Emerson Exchange 365

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PV Scale in Pid

On a simple PID control module where a pressure transmitter is controlling a valve output for pressure control we had an odd experience.  Historically the AI OUT_SCALE and XD_SCALE did not match the actual pressure transmitter scale. The module was updated to have the correct OUT_SCALE and XD_SCALE in the AI block and then set the PID PV_SCALE to match.  We made the change to all of these parameters to from 0-100, to 0-4000.  Upon changing these parameters the rate at which the PID out changed to control the valve became significantly slower.  The tuning parameters were not modified and the  production use was not changed.

Does the PV_SCALE get used in the calculations for the PID output ?  Or what would cause the sudden change in the execution of the block ? Also, this is all with Fieldbus devices.

mikesteiger

• 1 Mar 2022 7:49 PM
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3 Replies

• Junggyu Yu

  

  • 1 Mar 2022 11:23 PM

  Hi Mike
  
  In DeltaV PID equation, GAIN is normalized with the Scale. I have a similar experience. In my memory, the PID control became slower when the range was increase.

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• MPHymel

  

  • 2 Mar 2022 2:41 PM

  In reply to Junggyu Yu:

  Yes, in fact, you can take advantage of this by adjusting your gain by the same multiplier as your span. For instance (just last week, in fact) we changed a temperature controller range from 800-1100° to 200-100°. The span went from 300 to 900, increasing by a factor of 3. The controller gains were all adjusted by a factor of three and no re-tuning tuning was needed. Keep in mind that this doesn't affect your integral time.

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• mikesteiger

  

  • 3 Mar 2022 2:07 PM

  In reply to MPHymel:

  Thank you, this helps a lot.

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